This invention relates to a process for the manufacture of coatings for infomation discs. More particularly, this invention relates to the formation of optically transparent polymeric coatings for information discs which will prevent dust from coming into contact with the information disc. Although this invention will be described in terms of an information disc, it should be understood that the information storage device may have a shape other than that of a disc and such other shape is included in the scope of this invention.
Information storage discs may be prepared by using, for example, a laser to mark the surface of the disc with a retrievable information code. This marking may take the form of pits, a number of which may form a byte. In the interest of clarity and simplicity, the marks on the disc surface will be referred to as pits and the term "byte" may include a plurality of pits although the invention is not to be construed as limited to a marking which forms only pits. An information disc may contain, for example, several billion pits per disc. Each pit or series of pits represents a letter or a number. When the information is to be read from the disc, such reading is generally done by laser. The laser may be the same one which has been used to burn the information into the disc or may be a different laser. The laser uses converging light to read the pits of the disc. It therefore becomes important that the laser by focused properly so that the light will converge on the surfac eof the disc. If the laser is not focused properly or if a foreign object, such as a fine dirt particle, is on the surface of the disc then the pit will be obscured and will not be read by the laser. This will then affect the retrieval of information from the information disc. Because the laser uses converging light, it has become common practice to cover the surface of the information disc with an optically transparent polymeric protective layer to keep dust, and other foreign matter, from the surface of the disc. Because the reading laser utilizes converging light, which is wide as it enters the polymeric protective layer, any dust will not adversely affect the reading of the information disc since it will be focused past the dust on the protective layer and directly on the pit which is on the information disc.
The polymeric protective layer must itself be perfectly flat and optically transparent. If the polymeric protective coating is not flat, then the focus of the laser onto the disc surface is adversely affected.
When the polymeric protective layer is formed on the disc itself, the disc is rotated and a radiation curable coating is poured onto the information disc. Centrifical force throws the coating out toward the edges of the disc, and in this manner the disc is coated and the casting is then radiation cured.
The disadvantage of the aforedescribed proccess is that, because of the centrifical force, the coating assumes a wedge shape and develops waves, much as waves develop when a pebble is dropped into a still pool of water. In order to eliminate these waves, it is necessary to allow the coating to sit for a long period of time, uncured, on the disc which is now in a stationary position, to allow these waves to flow out. However, because of the centrifical force, the wedge, which was formed because of such force, collapses and forms its own waves which keep oscillating and which requires extended periods of time to flow out. Thus, there are two waves which are formed when the aforedescribed process is used. The first waves, which flow out quickly, are those which have been formed by the stream of liquid onto the disc surface and the second waves, which require a long period of time to dissipate, are those which have been formed due to collapse of the wedge shaped coating which has been formed through centrifical force.
Once all waves have dissipated from the disc, the coating is then exposed to radiation, such a UV light, and becomes a solid, flat, optically transparent coating.
It is an object of this invention to provide a process for forming a radiation curable coating onto an information disc.
Another object of this inveiton is to form said coating while minimizing the time required for the coating to become flat before curing.
other objects and advantages will become apparent from the following more complete description and claims.